1. Field of the Invention
The present invention relates to aqueous borohydride compositions, and its use as a hydrogen-based fuel, processes for converting such fuel into energy, and its use in purification of organic compounds, recovery of heavy and precious metals and de-colorization of wastewater streams, synthesis of pharmaceutical compounds, and the generation of sodium hydrosulfite in paper, leather and textile manufacture.
2. Brief Description of the Related Art
Fuel cells are known as a source of energy. Various types of fuel cells are known, including proton exchange membrane, molten carbonate, alkaline, phosphoric acid, and solid oxide. While technologies such as direct methanol processes have been investigated, such cells frequently employ hydrogen gas as a source of chemical energy, and convert that chemical energy into electricity for use in powering electric motor-driven vehicles and the like. The hydrogen gas may be obtained from water by electrolysis at a production facility, stored, and then transferred as a fuel to vehicles, and stored, for example, under high pressure in a suitable tank. This process has the obvious potential disadvantage of having to store quantities of hydrogen, a highly flammable material. Alternatively, the hydrogen gas can be generated in situ from another material. For example, hydrogen gas can be generated from natural gas using an on-board reformer, or from common gasoline using an autothermal reformer that extracts hydrogen from gasoline in a series of chemical conversion steps. In addition to organic compounds such as gasoline, another possible source of hydrogen are inorganic hydrogen compounds, such as metal hydrides. One example is sodium borohydride NaBH4. As an aqueous solution in a fuel cell, sodium borohydride reacts with water to liberate hydrogen in the presence of a suitable catalyst:NaBH4+2H2O=NaBO2+4H2
In conventional practice, an aqueous solution containing about 20% sodium borohydride and about 5% caustic soda in water is used as a propellant. This fuel is stable for a reasonable period of time; however, the high concentration of caustic in the conventional formulation shortens the effective life of the ruthenium catalyst, and creates environmental problems associated with the disposal or recycle of the spent sodium borohydride fuel formulation. In addition, the high pH fuel may adversely affect catalysts employed in decomposing the sodium borohydride to provide hydrogen or for the reaction of hydrogen with a suitable oxidant. It is clear, therefore, that the industry needs a safer, more efficient and more environmentally friendly propellant than the one offered by the conventional formulation. Thus, while fuel cell-related technology has rapidly progressed recently, there is a continuing need for stable, high quality fuels for use in such cells.
Sodium dithionite, also known as sodium hydrosulfite, is spontaneously ignitable and hence considered a hazardous material to transport. Nevertheless, sodium hydrosulfite is used in large quantities in the papermaking and textile industries. To avoid transporting sodium hydrosulfite, in recent years in situ preparation of sodium hydrosulfite from the reaction of sodium borohydride, sodium hydroxide solution and sodium bisulfite has been employed. Sodium borohydride is thus widely employed in the papermaking and textile industries to prepare sodium hydrosulfite, such as disclosed, for example, in U.S. Pat. No. 5,094,833, herein incorporated by reference. However, solid sodium borohydride is itself a pyrophoric material, which militates against its use in industrial processes. On the other hand, conventional aqueous solutions of sodium borohydride are not pyrophoric, but tend to have very limited stability unless stabilized with a caustic material. The water of solution disadvantageously increases the cost of shipping borohydride, while the caustic employed for stabilization requires additional steps for post-treatment disposal. Thus, there is a need for a stable, low alkaline liquid form of sodium borohydride in many conventional applications where sodium borohydride is used today including recovery of heavy and precious metals, purification of organic alcohols and amines, synthesis of pharmaceutical compounds, and the generation of sodium hydrosulfite for the textile, leather and paper industries.